Apparatus and method for recording an information on a recordable optical record carrier using oval spot profile

ABSTRACT

The present invention relates to a method and a corresponding apparatus for recording an information on a recordable optical record carrier ( 2 ) by irradiation of a light beam through optical means ( 3 - 7 ) onto said record carrier ( 2 ) for forming marks and lands representing said information along an information recording direction (t). To obtain a higher recording density, and thus higher data capacity, it is proposed according to the present invention to use astigmatism for influencing the light beam from the light source ( 8 ) to said record carrier ( 2 ) during recording of information so as to obtain a light beam having a substantially oval spot profile having a shorter axis in the information recording direction (t), i.e. the tangential direction for an optical disc, compared to the direction (r) orthogonal that to, i.e. the radial direction.

The present invention relates to a an apparatus for recording aninformation on a recordable optical record carrier by irradiation of alight beam onto said record carrier for forming marks and landsrepresenting said information along an information recording direction.The present invention relates further to a corresponding recordingmethod, an optical record carrier and a computer program forimplementing said method.

The total data capacity of optical discs is determined by the radial andtangential data density. The radial density is determined by the datatrack pitch, the tangential data capacity by the shortest mark that canbe written. For rewritable phase-change discs, re-crystallization at thetrailing edge of a mark during writing of the next mark is used toobtain a mark of shorter length than the optical spot size. Typically,half of the mark is erased to end up with a crescent shaped mark. Forthe recently introduced Blu-ray Disc (BD), a total data capacity of 25Gbyte can be recorded on a single recording layer of a 12 cm disc. Theshortest length is 150 nm (d=1 code) while the optical spot is 300 nm inwidth (1/e radius of the optical spot is 150 nm).

A recordable (write-once) BD format is required in addition to the RW(rewritable) format. This so-called BD-R system should also achieve thehigh data-capacity of a BD-RW system, i.e. 25 GB, to allow one-to-onecopies. In recordable media mark formation occurs when a thresholdtemperature is exceeded. The mark-formation corresponds to anirreversible change in the information layer, and effects comparable tothe re-crystallization in rewritable media in principle do not occur.Thus, the marks that have been recorded in recordable media essentiallyreflect the optical/thermal profile that was present during therecording process. To reduce the mark length, thus to increase thetangential density, less laser power may be used to write a smallermark. As a consequence, the modulation of these smaller marks, and thussignal-to-noise ratio, will drop as well. This can be similarlydiscussed for write-once media based on phase-change material asrecording material. The as-deposited amorphous material willre-crystallize to form circularly shaped crystalline marks. Mark lengthreduction due to post heat is not possible.

U.S. Pat. No. 5,673,246 discloses an optical pick-up device which makesa numerical aperture of an object lens in a radial direction of anoptical disc smaller than that in a tangential direction to a track onthe optical disc. Thus, a spot diameter in the tangential direction tothe track becomes smaller than that in the radial direction duringread-out. Thus, it is possible to prevent degradation MTF (ModulationTransfer Function) as well as to accurately read data recorded on thedisc.

It is therefore an object of the present invention to provide arecording apparatus and a corresponding recording method for recordingan information on a recordable optical record carrier by which datacapacities can be achieved that are similar to the data capacities ofcorresponding rewritable optical record carriers.

This object is achieved according to the present invention by arecording apparatus as claimed in claim 1, comprising:

-   -   a light source for generating a light beam,    -   optical means for irradiating said light beam onto said record        carrier,        wherein said optical means comprise means for influencing said        light beam from said light source to said record carrier during        recording of information by use of astigmatism so as to obtain a        light beam having a substantial oval spot profile having a        shorter axis in the information recording direction compared to        the direction orthogonal thereto.

This object is further achieved according to the present invention by acorresponding recording method as claimed in claim 9.

The present invention is based on the idea to use a non-circularspot-profile for the recording process. If the spot-profile is elongatedin the radial direction, an oval-like thermal profile on the recordinglayer is obtained, so that marks can be written that are narrow intangential direction compared to their radial extent thus increasing theresolution. Such an oval spot profile is obtained according to thepresent invention by use of astigmatic aberration of the wavefront ofthe light beam. As a result of astigmatism two focal ‘lines’ (ovals) areformed. By choosing the proper direction of the astigmatism duringrecording, an oval spot profile with the long axis in the radialdirection is obtained. No optical power is thereby lost.

The proposed spot modification is also beneficial for high-speedwrite-once recording. At high recording speeds, thermal in-trackinterference may hamper the pit formation process. To reduce the thermalin-track interference, an ellipsoidal optical spot, such as obtainedwith the proposed modification, leads to less direct heating of thepreviously written pits.

Preferred embodiments of the invention are defined in the dependentclaims. According to a preferred embodiment an astigmatism can beintroduced into the light beam deliberately, for instance by us of aliquid crystal means, such as a liquid crystal cell, or by use of asimple cylindrical lens. Preferably, the liquid crystal cell fills acylindrical-lens-shaped cavity. When properly designed, the applicationof electrical voltage will make the liquid crystal molecules to changetheir orientation. This then results in a change of refractive indexwhich allows the ‘lens’ to be turned on and off. In the off-state therefractive index of the liquid crystal should match that of the cell, sothat no optical effect is introduced.

Preferably, the cell is oriented tangentially or radially compared tothe grooves on the disc. The best solution is to put the cell such thatlight in radial direction is affected by the liquid crystal cell. If theliquid crystal cell is off, light is focused on the disc resulting in a(nearly) circular spot. If then the liquid crystal cell is switched on,rays in radial direction get a little bit defocused due to thecylindrical lens resulting in the desired oval spot-shape, radiallybroadened, narrower in tangential direction. It is also possible to usea cell which affects the tangential direction, but when it is thenturned on, some re-focusing is required to get the desired spot-shape.

According to an alternative embodiment the focus position of the focallines of the light beam having an intrinsic astigmatism can becontrolled such that a defocus is introduced during recording ofinformation. Such an intrinsic astigmatism is generally produced by thelight source used, in particular by a laser diode itself. For instance,edge-emitting laser diodes always have some intrinsic astigmatism due tothe internal shape of the device, i.e. the lasermode is not fullyconfined to the optical gain region. This astigmatism only has theproper direction if the output of the laser is placed tangential orradial with respect to the grooves on the disc. The advantage is that noadditional elements are required in the recording device.

Finally, the proper direction of the spot on the disc is controlled bythe focus-offset. Both oval-directions can be obtained albeit atdifferent focal distances, e.g. different focus-offset. In an opticaldrive the disc is kept into focus by electronic means: the height of thelens above the disc is controlled by actuators. The current through theactuators is such as to minimize a so-called focus-error signal.Deliberate defocus can be introduced by adding e.g. an offset to thefocus error signal. Such control means are present in existing drivesfor the purpose of active focussing which are adapted according to thepresent invention for recording by use of an oval spot profile.

In addition, appropriate control means are provided for switching saidmeans for influencing the light beam on or off or for bringing saidmeans into the light path during recording and for removing itthereafter, in particular during read-out or RW-recording wherespot-deformation is not desired. For instance, a cylindrical lens can beeasily ‘turned on and off’ by mechanical placement of this lens in andout of the beam. Another possibility is to electrically switch a liquidcrystal cell on or off.

The proposed solution can also be helpful in high-speed recording. Thereason for this is that at high-speed recording similar problems arefound as with high-density recording. In high-speed recording subsequentmarks are written with very little timing-delay because of the highspeed. A consequence is that neighboring marks are affecting each othervia so-called thermal interference: heat present from the previous(next) mark influences the formation of the next (previous) mark. Byusing a spot that is narrower in tangential direction such effects canbe reduced.

A computer program comprising computer program means for crossing acomputer to perform the steps of the method as claimed in claim 9 whensaid computer program is run on a computer is defined in claim 10.

The present invention will now be explained in more detail withreference to the drawings in which

FIGS. 1 a, 1 b show long and short marks recorded on rewritable andrecordable record carriers with a circular spot profile,

FIGS. 2 a, 2 b show long and short marks recorded with a circular and anoval spot profile on recordable record carriers,

FIGS. 3 a, 3 b show an optical disc according to the present invention,

FIG. 4 illustrates the effect of astigmatism,

FIG. 5 shows a first embodiment of a recording apparatus according tothe present invention,

FIG. 6 shows a second embodiment of a recording apparatus according tothe present invention,

FIG. 7 shows a third embodiment of a recording apparatus according tothe present invention,

FIG. 8 shows the optical spot profile used for read-out of data,

FIG. 9 shows the optical spot profile used for recording of data and

To achieve data capacities in write-once systems that are similar to thedata capacities of corresponding rewritable systems, very narrow (intangential direction t) marks need to be written at short runlengths,while in rewritable media the short marks are as broad (in radialdirection r) as the long marks. In recordable media the shortest marksare as broad as their lengths, i.e. have a circular shape. Thus, thewidth of the shortest marks decreases resulting in lower modulationduring read-out. This is illustrated in FIG. 1 showing a schematicdrawing of long (8T) and short (2T) marks recorded in rewritable (RW)media (FIG. 1 a) and recordable (R) media (FIG. 1 b) at high density. Itshall be noted that the shortest marks are comparable or smaller thanthe optical spot profile. The consequence is that during read-out, theoptical modulation of the shortest marks in recordable media vanishes.

This problem can also be illustrated as well by comparing the resolution(=2T_(pp)/8T_(pp)) of the 2T marks for BD-RW and BD-R at a density of 23GB. According to the BD-RW specification, the 2T resolution should belarger than 10%, and this is met in practice. However, so far even inthe best BD-R media the resolution at 23 GB is at most 5%. This poorresolution significantly deteriorates the jitter. The current inventionprovides a solution to write small but broad marks, i.e. having a hightangential density, such that a high modulation is preserved.

FIG. 2 shows a schematic drawing of long (8T) and short (2T) marks bothrecorded on a recordable medium. The marks shown in FIG. 2 a have beenrecorded using a circular spot shown on the right-hand side while themarks shown in FIG. 2 b have been recorded using an oval spot shown atthe right-hand side, said oval spot having a shorter axis in thetangential direction t compared to the axis in the radial direction r.As can be seen the marks shown in FIG. 2 b are narrow in the tangentialdirection t compared to their radial extent and compared to the marksshown in FIG. 2 a. Thus, the desired increase of the resolution can beobtained.

FIG. 3 shows a recordable optical disc according to the presentinvention in a top-view (FIG. 3 a) and in a cross-sectional view (FIG. 3b). The disc 2 comprises a number of guide grooves 13, which indicatethe direction in which the information is recorded on the disc 2, i.e.the information is recorded along said spiral grooves 13. Indicated arefurther in FIG. 3 the tangential direction t and the radial direction r.

FIG. 3 b shows as cross-sectional view the different layers of the disc2 which, in general, comprises a substrate layer 2 a, a recording layeror recording stack 2 b and a cover layer 2 c.

FIG. 4 illustrates the effect of astigmatism. In it's simplest form,astigmatism means that for a focused beam of light travelling in thez-direction, the focal distance of the beam is different in twoperpendicular directions, e.g. x- and y-directions. Astigmatism can beachieved by putting a cylindrical lens C in the light-path L. Thiscylindrical lens C will make light in one direction, for instance x, tobe bended, but it will not affect the light in the y-direction. Ifbehind this cylindrical lens C a normal (spherical) lens S, e.g. theobjective lens of an optical recorder, is placed the result will be thatlight in the x-direction will be focused first, which was alreadysomewhat bended by the cylindrical lens C, while the y-direction is notyet fully focused. This will result in a line-shaped (or oval) focus Ox.At a somewhat further distance, the y-direction will be in completefocus but at the same time the x-direction is already beyond focusresulting again in a line-shaped (oval) focus Oy perpendicular to thefirst one.

A first embodiment of a recording apparatus according to the presentinvention is schematically shown in FIG. 5. During recording or read-outthe disc 2 is rotated by rotating means 1. A light beam generated by alight source, in particular a laser diode 8, is focused onto the disc byan objective lens 3 after the light beam has passed a collimator lens 7,a liquid crystal cell 14, a polarizing beam splitter 6, a quarter-waveplate 5 and a diaphragm 4 for aperture limitation. During read-out thepolarizing beam splitter 6 reflects light reflected from the disc 2through a lens 9 on a photo detector (array) 10 for analyzing thereflected light. The general function of such a recording apparatus iswidely known and shall therefore not be explained further here.Switching of the liquid crystal cell 4 can be done electrically.

The liquid crystal cell fills a cylindrical-lens-shaped cavity. Whenproperly designed, the application of electrical voltage will make theliquid crystal molecules to change their orientation. This then resultsin a change of refractive index which allows the ‘lens’ to be turned onand off. In the off-state the refractive index of the liquid crystalshould match that of the cell, so that no optical effect is introduced.Preferably, the liquid crystal cell is radially oriented so that lightin radial direction is affected.

Instead of the liquid crystal cell 14 a cylindrical lens could be placedin the light beam as well which can be mechanically placed in and out ofthe light beam. Further, either the liquid crystal cell 14 or thecylindrical lens can also be placed between the quarter-wave plate 5 andthe polarizing beam splitter 6 as shown in FIG. 6.

FIG. 7 shows another embodiment of a recording apparatus according tothe present invention. Therein focus control means 15 are provided forcontrol of the height of the objective lens 3 above the disc 2, e.g. byappropriate actuators (not shown). The current through the actuators iscontrolled so that a so-called focus-error signal is minimized.

In this embodiment use is made of the intrinsic astigmatism which isgenerally produced by the light source 8 used, in particular by a laserdiode itself. In case of recording the focus control means 15 can be(partially) disabled, for instance electrically or mechanically, or adefocus can be deliberately introduced by adding, e.g. an offset to thefocus error signal during recording of information by an offset settingmeans 16 which can be turned on and off.

The focussed optical spot profile and marks/pits obtained by therecording method according to the present invention in a groove G (13)separated from another groove G by a land area L are shown in FIG. 8.During reading along the information recording direction t (=tangentialdirection) a circular optical spot profile 11 a is used. Shown are alsothe recorded marks 12 having an oval profile.

FIG. 9 shows the optical spot profile 11 b used for recording of data.As can be seen an oval spot profile 11 b is used resulting in oval marks12.

The present invention provides a solution to obtain higher datacapacities when recording in particular on recordable optical recordcarriers. Astigmatism is used for influencing the light beam from thelight source to the record carrier during recording of informationaccording to the present invention to obtain a light beam having asubstantially oval spot profile during recording, in particular having asmaller width in the tangential direction compared to the width inradial direction.

1. A recording apparatus for recording an information on a recordableoptical record carrier by irradiation of a light beam onto said recordcarrier for forming marks and lands representing said information alongan information recording direction, comprising: a light source forgenerating a light beam, optical means for irradiating said light beamonto said record carrier, wherein said optical means comprise means forinfluencing said light beam from said light source to said recordcarrier during recording of information by use of astigmatism so as toobtain a light beam having a substantial oval spot profile having ashorter axis in the information recording direction compared to thedirection orthogonal thereto.
 2. A recording apparatus as claimed inclaim 1, wherein said means for influencing the light beam are adaptedfor introduction of astigmatism into the light beam.
 3. A recordingapparatus as claimed in claim 2, wherein said means for influencing thelight beam comprise a liquid crystal cell.
 4. A recording apparatus asclaimed in claim 3, wherein said liquid crystal cell has a cylindricalshape.
 5. A recording apparatus as claimed in claim 2, wherein saidmeans for influencing the light beam comprise a cylindrical lens.
 6. Arecording apparatus as claimed in claim 1, wherein said means forinfluencing the light beam comprise a focus control means for control ofthe focus position of the focal lines of the light beam having anintrinsic astigmatism such that a defocus is introduced during recordingof information.
 7. A recording apparatus as claimed in claim 1, whereinsaid focus control means are adapted for adding an offset to a focuserror signal used for keeping the optical into focus during recording ofinformation.
 8. A recording apparatus as claimed in claim 1, furthercomprising a control means for control of said means for influencing thelight beam by switching said means on or off by bringing said means intothe light path during recording.
 9. A method of recording an informationon a recordable optical record carrier by irradiation of a light beamthrough optical means onto said record carrier for forming marks andlands representing said information along an information recordingdirection, wherein the light beam from a light source to said recordcarrier during recording of information is influenced by making use ofastigmatism so as obtain a light beam having a substantially oval spotprofile having a shorter axis in the information recording directioncompared to the direction orthogonal thereto.
 10. Computer programcomprising computer program means for causing a computer to perform thesteps of the method as claimed in claim 9 when said computer program isrun on a computer.